Field effect transistors, such as a thin film transistor (TFT), are widely used as the unit electronic device of a semiconductor memory integrated circuit, a high frequency signal amplification device, a device for a liquid crystal drive, or the like, and they are electronic devices which are currently most widely put into practical use. Of these, with significant improvement in displays in recent years, in various displays such as a liquid crystal display (LCD), an electroluminescence display (EL) and a field emission display (FED), a TFT is frequently used as a switching device which drives a display by applying a driving voltage to a display device.
As a material of a semiconductor layer (channel layer) which is a main component of a field effect transistor, a silicon semiconductor compound is used most widely. Generally, a silicon single crystal is used for the high frequency amplification device or the device for integrated circuits which need high-speed operation. On the other hand, an amorphous silicon semiconductor (amorphous silicon) is used for a device for driving a liquid crystal in order to satisfy the demand for realizing a large-area display.
A thin film of amorphous silicon can be formed at relatively low temperatures. However, the switching speed thereof is slow as compared with that of a crystalline thin film. Therefore, when it is used as a switching device which drives a display, it may be unable to follow the display of a high-speed animation. Specifically, amorphous silicon having a mobility of 0.5 to 1 cm2/Vs could be used in a liquid crystal television of which the resolution is VGA. However, if the resolution is equal to or more than SXGA, UXGA and QXGA, a mobility of 2 cm2/Vs or more is required. Moreover, if the driving frequency is increased in order to improve the image quality, a further higher mobility is required.
As for a crystalline silicon-based thin film, although it has a high mobility, there are problems that a large amount of energy and a large number of steps are required for the production, and that a large-sized film formation is difficult. For example, when a silicon-based thin film is crystallized, laser annealing which requires a high temperature of 800° C. or more or expensive equipment. In the case of a crystalline silicon-based thin film, the device configuration of a TFT is normally restricted to a top-gate configuration, and hence, reduction in production cost such as decrease in the number of masks is difficult.
In order to solve the problem, a thin film transistor using an oxide semiconductor film formed of indium oxide, zinc oxide and gallium oxide has been studied. In general, an oxide semiconductor thin film is formed by sputtering using a target (sputtering target) composed of an oxide sintered body.
A target having a crystal structure of a homologous crystal structure compound such as In2Ga2ZnO7 and InGaZnO4 is known (Patent Documents 1 to 3). However, in this target, in order to increase the sintering density (relative density), it is required to conduct sintering in an oxidizing atmosphere. In this case, in order to reduce the resistance of the target, a reduction treatment at a high temperature is required to be conducted after sintering. Further, if the target is used for a long period of time, problems arise that the properties of the resulting thin film or the film-forming speed largely change; abnormal discharge due to abnormal growth of InGaZnO4 or In2Ga2ZnO7 occurs; particles are frequently generated during film formation or the like. If abnormal discharge occurs frequently, plasma discharge state becomes unstable, and as a result, stable film-formation is not conducted, adversely affecting the film properties.
Patent Document 4 discloses a sputtering target formed of Ga-doped indium oxide. However, a sputtering target disclosed in Patent Document 4 which are obtained by doping indium oxide with Ga containing 100 at. ppm of a metal having a valency of positive tetravalency or higher suffers from a variation in target density, and hence, it is difficult to produce the target such that it has a relative density of 97% or more.
Further, Patent Document 5 discloses a sputtering target of Al-doped indium oxide. In this sputtering target, the atomic ratio of Al is 0.001 to 45%, which means a wide composition ratio. In addition, the ratio of ions to be doped with the target having a valency of positive tetravalency of higher is wide, i.e. 10 to 5000 at. ppm. Therefore, the optimum composition region as the oxide semiconductor is not clear.
As mentioned above, studies to be made on the target when forming the oxide semiconductor film by the sputtering method are not enough.